Digestive and absorptive functions of the small intestines

Question 1

What are the nutritionally important carbohydrates?

Answer 1

Starch (amylose & amylopectin), disaccharides (sucrose, lactose), monosaccharides (glucose, galactose, fructose), and dietary fibers.

Question 2

How are glucose and galactose absorbed?

Answer 2

Via secondary active transport using SGLT-1 (sodium-glucose cotransporter).

Question 2

Salivary amylase and pancreatic amylase.

Answer 2

Maltose (2 glucose units), maltotriose (3 glucose units), and α-limit dextrins (5-6 glucose units).

Question 2

What enzymes are involved in luminal digestion of carbohydrates?

Answer 2

Salivary amylase and pancreatic amylase.

Question 2

What is the role of pepsin in protein digestion?

Answer 2

Pepsin digests dietary proteins at neutral amino acids in the stomach (prefers low pH).

Question 3

How is fructose absorbed?

Answer 3

By facilitated diffusion via GLUT5 transporter.

Question 4

What activates trypsinogen to trypsin?

Answer 4

Enterokinase in the duodenum.

Question 5

How are di- and tripeptides absorbed?

Answer 5

Via PEPT1 transporter coupled with H⁺ ions.

Question 6

What are the four systems for amino acid absorption?

Answer 6

Neutral, basic, proline/glycine, and dicarboxylic (glutamic/aspartic acids).

Question 6

What is the role of bile salts in lipid digestion?

Answer 6

Emulsify fats and form micelles for absorption.

Question 6

How are monoglycerides and FFAs absorbed?

Answer 6

Passive diffusion from micelles into enterocytes.

Question 7

What happens to long-chain FFAs (>12 carbons) after absorption?

Answer 7

Re-esterified into triglycerides and packaged into chylomicrons for lymphatic transport.

Question 8

What is the role of colipase?

Answer 8

What is the role of colipase?

Question 9

How is vitamin B₁₂ absorbed?

Answer 9

Bound to intrinsic factor (IF) in the ileum via IF-cobalamin receptors.

Question 10

What enhances iron (Fe²⁺) absorption?

Answer 10

Gastric acid (reduces Fe³⁺ to Fe²⁺) and ascorbic acid (vitamin C).

Question 11

How is calcium absorption regulated?

Answer 11

Active transport stimulated by vitamin D₃; inhibited by phosphates/oxalates.

Question 11

What causes lactose intolerance?

Answer 11

Decline in brush border lactase enzyme, leading to undigested lactose fermented by colonic bacteria (bloating, diarrhea).

Question 12

What is celiac disease?

Answer 12

Autoimmune reaction to gluten (α-gliadin), damaging intestinal villi and causing malabsorption.

Question 13

What is glucose-galactose malabsorption?

Answer 13

Mutation in SGLT-1 transporter, causing severe diarrhea; treated with fructose-substituted diets.

Question 13

How does cholera toxin cause diarrhea?

Answer 13

Activates adenylate cyclase → ↑cAMP → ↑Cl⁻ secretion and ↓Na⁺ absorption in the intestine.

Question 14

Where is most water reabsorbed in the GI tract?

Answer 14

Jejunum (5500 ml/day), ileum (2000 ml/day), and colon (1300 ml/day).

Question 14

How does aldosterone affect Na⁺ absorption?

Answer 14

Enhances Na⁺ reabsorption in the ileum and colon by upregulating Na⁺/K⁺ ATPase.

Question 15

What is the role of CFTR in secretory diarrhea?

Answer 15

CFTR channel secretes Cl⁻ into the lumen, followed by Na⁺ and water (e.g., in cholera).

Question 15

Which enzyme digests amylose and amylopectin, and what are its products?

Answer 15

Pancreatic amylase; produces maltose, maltotriose, and α-limit dextrins.

Question 15

What brush border enzymes digest disaccharides?

Answer 15

Sucrase (sucrose → glucose + fructose), lactase (lactose → glucose + galactose), maltase (maltose → 2 glucose).

Question 15

What is the role of enterokinase?

Answer 15

Activates trypsinogen → trypsin in the duodenum, initiating protein digestion.

Question 16

Which pancreatic enzyme hydrolyzes triglycerides at positions 1 and 3?

Answer 16

Pancreatic lipase (requires colipase to function in the presence of bile salts).

Question 16

What enzyme breaks down cholesterol esters?

Answer 16

Cholesterol esterase (also digests triglycerides and vitamin esters).

Question 17

How are small peptides absorbed?

Answer 17

Via PEPT1 (H⁺-coupled cotransporter) into enterocytes.

Question 17

What transporter absorbs vitamin C, and how is it regulated?

Answer 17

SVCT1/SVCT2 (Na⁺-dependent); downregulated by high vitamin C intake.

Question 18

How does fructose absorption differ from glucose/galactose?

Answer 18

Fructose uses GLUT5 (facilitated diffusion), while glucose/galactose use SGLT-1 (active transport).

Question 19

What happens if SGLT-1 is defective?

Answer 19

Glucose-galactose malabsorption → osmotic diarrhea, managed with fructose diets.

Question 20

How are nucleic acids absorbed?

Answer 20

Nucleases → nucleotides → nucleosides + phosphate → sugars + bases (absorbed actively).

Question 21

What is the composition of a micelle?

Answer 21

Bile salts, monoglycerides, FFAs, cholesterol, and fat-soluble vitamins.

Question 21

Why are micelles critical for fat absorption?

Answer 21

Solubilize hydrophobic lipids for diffusion across the unstirred water layer.

Question 21

What happens to absorbed long-chain FFAs?

Answer 21

Re-esterified into triglycerides → packaged into chylomicrons → lymphatics.

Question 22

Why do pancreatic insufficiency patients have steatorrhea?

Answer 22

Lack of lipase and colipase → impaired micelle formation → fat malabsorption.

Question 22

What causes lactose intolerance symptoms?

Answer 22

Undigested lactose → bacterial fermentation → gas (H₂/CO₂), osmotic diarrhea.

Question 22

How is celiac disease managed?

Answer 22

Strict gluten-free diet (avoid wheat/barley/rye) to prevent villous atrophy.

Question 22

What is Hartnup disease?

Answer 22

Defect in neutral AA transport (e.g., tryptophan) → pellagra-like symptoms; treated with nicotinamide.

Question 22

Why does gastrectomy cause iron deficiency anemia?

Answer 22

Loss of gastric acid → impaired reduction of Fe³⁺ to absorbable Fe²⁺.

Question 22

How does cholera toxin disrupt fluid balance?

Answer 22

↑cAMP → CFTR activation → Cl⁻ secretion → Na⁺/water follow → secretory diarrhea.

Question 23

What is the pathophysiology of cystic fibrosis-related malabsorption?

Answer 23

Thick mucus blocks pancreatic enzyme secretion → fat/protein maldigestion.

Question 24

Why are oral rehydration solutions (ORS) effective?

Answer 24

Glucose enhances Na⁺ absorption via SGLT-1 → osmotic water reabsorption.

Question 25

Where is most NaCl absorbed?

Answer 25

Jejunum (nutrient-coupled) and colon (aldosterone-dependent electrogenic absorption).

Question 25

Why are breast milk lipases important for neonates?

Answer 25

Compensate for immature pancreatic function (e.g., bile salt-stimulated lipase).

Question 25

What inhibits calcium absorption?

Answer 25

Phosphates, oxalates (form insoluble salts); enhanced by lactose/vitamin D₃.

Question 26

How is vitamin B₁₂ transported in blood?

Answer 26

Bound to transcobalamin II (defects cause megaloblastic anemia).

Question 26

How does the colon secrete bicarbonate?

Answer 26

Cl⁻/HCO₃⁻ exchanger (pendrin) at the apical membrane; driven by basolateral Na⁺/K⁺ ATPase.

Question 27

What is the role of NHE3 in the intestine?

Answer 27

Apical Na⁺/H⁺ exchanger (electroneutral NaCl absorption in small intestine/colon).